Geared turbofan engines utilize a gas powered turbine engine to drive rotation of a fan through a gearing system. The gas powered turbine engine includes a compressor that draws in and compresses air, a combustor that mixes compressed air with a fuel and ignites the mixture, and a turbine across which the combustion products are expanded. The expansion of the combustion products across the turbine drives the turbine to rotate. Rotation of the turbine is communicated to the compressor via a shaft and drives rotation in the compressor as well. The rotation of the shaft is translated through the gearing system to the fan, which is positioned fore of the compressor.
When a geared turbofan engine is shut down while on the ground, such as when the aircraft in which the geared turbofan is installed is parked at an airfield, crosswinds entering the geared turbofan can drive rotation of the fan. The fan rotation can be either clockwise or counterclockwise, dependent on the direction of the wind, relative to the forward face of the aircraft engine. This rotation is referred to as windmilling.
Rotation of the fan, in turn, is translated to the shaft through the gearing system. In some geared turbofans, one or more components within the engine require active lubrication while the engine is rotating in order to prevent damage or deterioration to the engine systems.